The present invention relates generally to the field of radar detectors.
European and perhaps worldwide 5 GHz WLANs (Wireless Local Area Networks) and other radio applications are being required to detect radars and change channels if a radar is present. This allows the WLANS or other radio applications to share frequencies currently used only by radars, without interfering with radar operation. In general, if a WLAN or other radio device operates in a frequency band shared with a radar application, the device is desirably configured to determine whether an active radar signal is present in the band, and to transmit only when no active radar signal is present.
Some requirements for radar detection are specified in developing standards documents, such as ITU-R M. [8A-9B/RLAN-DFS] and ETSI EN 300 893 which are being reflected in IEEE 802.11h, an amendment to the IEEE 802.11a WLAN standard. These documents assume nearly perfect and immediate radar detection. The standards require a 60-second observation period before starting WLAN operations on a new frequency. This observation period could result in a wireless network outage of several minutes if a radar is detected on the current frequency and has to check several other frequencies before finding one that is unoccupied. This means that the detector should never indicate a radar is using the frequency when no radar is actually present (a false alarm) because it will lead to intolerable network outages.
The demands on the radar detector are thus very high. Ideally, the radar detector should have nearly 100% probability of detecting a radar, with nearly 0% possibility of false alarm.
The key characteristics of radars that will distinguish them from other signals are that they send out pulses of radio energy of nominally equal duration at nominally constant repetition periods. Thus the presence of pulses all having the same pulse width and the same time period between pulses is a good indicator of the presence of a radar. Different types of radar may have different pulse widths or periods, appropriate to their specific function.
One radar characteristic that causes detection difficulties is scanning. Many radars use a rotating antenna to observe a 360-degree view. This means that the WLAN may receive the radar only when the radar""s antenna is pointing directly at the WLAN location. This will be for some finite time duration, depending on the rate of rotation and the antenna type. Several of the types of radars that may be using the band are expected to be received by a WLAN device only long enough to see five pulses. This means that the radar detector used by the WLAN device should detect the radar with nearly 100% probability when it sees just five radar pulses.
The standards further complicate the radar-detection problem by stipulating that radar detectors must work despite the presence of normal WLAN transmissions. These transmissions may make it difficult for the radar detector to see the radar pulses, or may prevent pulse detection altogether. If the radar detector is located within a unit that is itself transmitting normal WLAN signals or other radio signals, the radar detector will not be able to receive radar pulses during those WLAN transmissions. The ETSI document specifies a transmit/quiet duty cycle while the ITU-R document specifies a series of randomly selected WLAN signals and quiet periods. Either of these test conditions can result in legitimate radar pulses going undetected.
Thus, a radar detector should determine whether or not a radar is present by detecting as many of the radar pulses as it can, perhaps out of a total of as few as only five pulses. Some of the radar pulses may not be detected due to normal reception or transmission of WLAN signals. These missing pulses could cause errors in measurements of the time between pulses, which would degrade the detection probability.
FIG. 1 shows a block diagram of a prior art radar detector, and in particular a periodicity validator portion of the radar detector. The periodicity validator is the portion of the radar detector that indicates when a new pulse matches an expected pulse period.
The new pulse period is measured by subtracting the time the last pulse was received from the time the new pulse was received. The new pulse period is compared to an expected pulse period. The expected pulse period is determined from evaluating the duration which occurred between at least two previously-received pulses. The prior art circuit shown in FIG. 1 is usually inactive until two pulses have been received and the time between them has been measured to determine an expected pulse period.
The absolute value of the difference between the new pulse period and the expected pulse period is compared to a desired period-match accuracy. The output of this comparison is 1 if the new-pulse period matches the expected pulse period to within the desired accuracy. If the desired accuracy is not met, the output of the comparison is 0. At the output of the circuit, a 1 indicates that the new pulse period matches the expected period, while a 0 indicates that the pulse does not match the expected period. The radar detector uses this information as part of its detection algorithm. Usually, a predetermined number of valid new pulses must be received before an active radar signal is identified.
This prior art radar detector requires the pulses to be received in consecutive order. Moreover, the number of consecutive pulses that must be received must be at least three, with two of the three pulses being used to establish a period and a third to confirm the period. Unfortunately, the requirement of detecting three consecutive pulses leads to an excessive false-negative rate. The false-negative rate represents the rate at which the radar detector falsely indicates that no radar is present when in fact a radar is present. And, the already-excessive false-negative rate increases further if the number of consecutive pulses that must be received is greater than three.
It is an advantage of one embodiment of the present invention that an improved radar detector having a multi-period periodicity validator is provided.
Another advantage of one embodiment of the present invention is that a periodicity validator is provided which accounts for missing pulses.
Still another advantage of one embodiment of the present invention is that a pulse evaluator is provided which achieves a suitable false-negative rate because no limitation is imposed regarding the receipt of consecutive radar pulses.
Yet another advantage of one embodiment of the present invention is that a plurality of potential radar pulse trains may be tracked simultaneously.
These and other advantages are realized in one form by an improved radar detector having a multi-period periodicity validator. The radar detector includes a pulse sensor configured to sense a received pulse. A first-period detector couples to the pulse sensor and is configured to detect a first period. A second-period detector couples to the pulse sensor and is configured to detect a second period. The second period is different from the first period. A control element couples to the first-period detector and to the second-period detector. The control element determines if a pulse sensed at the pulse sensor exhibited a period matching one of the first and second periods.